1. Field of the Invention
The present invention relates generally to the preparation of structural ceramic materials, and more particularly to a process for producing reinforced ceramic composites including the introduction of retractory barrier interface layers between successive layers of a ceramic matrix material.
Structural ceramic materials are utilized in a variety of high temperature applications which may also require high strength. For example, structural ceramics are utilized as high temperature insulating materials in the construction of rocket motors and as fan blades, combustion chambers, and after burners in the construction of jet engines.
Heretofore, structural ceramics have usually been produced by one of two processing techniques. In a first technique, referred to as hot press sintering, ceramic powders, such as silicon carbide, alumina, boron nitride, and the like, are subjected to very high pressures and temperatures, typically in the range from about 1500.degree. C. to 2100.degree. C. While the ceramics produced by such hot press processes are generally satisfactory, the high temperatures required will usually degrade any reinforcement materials which may be present in the ceramic. For that reason, hot press ceramics are normally not reinforced and therefore possess limited fracture toughness and strength. Moreover, the high processing temperatures require the use of expensive tooling which increases the capital costs associated with the hot press process.
Reinforced ceramic composites may also be prepared by chemical vapor deposition (CVD) of a ceramic percursor material onto a fiber reinforcement substrate. The precursor material is converted to the corresponding ceramic material by conventional techniques, and the process has the advantage that the reinforcement fibers are not exposed to excessively high temperatures which can result in degradation. The process, however, also has disadvantages. In particular, the CVD process results in a non-uniform penetration of the reinforcement substrate, leading to non-uniform properties in the resulting composite material. Also, the matrix material resulting from successive application and heat treatment of the composite is often brittle with a propensity toward crack propagation, potentially leading to catastrophic failure if the material is overstressed.
For the above reasons, it would be desirable to provide efficient and economical processes for producing reinforced ceramic materials having fracture toughness and high flexural and tensile strength. Such processes should avoid subjecting the materials to very high temperatures capable of degrading the reinforcement fibers and should not require the use of expensive tooling. Moreover, the matrix material in the ceramics should be tough and resistant to crack propagation, even when subjected to high repeated stress.
2. Description of the Background Art
U.S. Pat. No. 4,546,163 discloses the pyrolytic conversion of vinyl-containing polysilanes to silicon carbide, particularly for the preparation of fibers. U.S. Pat. Nos. 4,414,403; 4,472,591; and 4,497,787, each disclose the preparation of silicon carbide by pyrolysis of certain branched polycarbonsilanes, which may include vinyl groups. The use of chemical vapor deposition of organosilicon compounds to Additionally, the high pressures required for sintering greatly complicate the formation of complex shapes and degrade the microstructure of the resulting ceramic, increasing the chance of product delamination. form silicon carbides is disclosed in U.S. Pat. Nos. 4,492,681, and 4,560,589. The preparation of silicon carbide by pyrolysis of a variety of substituted polycarbosilanes is taught in U.S. Pat. Nos. 4,310,481; 4,310,482; 4,314,956; and Re. 31,477. These patents further teach that the polycarbosilanes may be coated on articles prior to pyrolysis and conversion to silicon carbide.